Well...here's the thing.
Jonnas claims to have targeted the person killed last night which is suspicious in his own right. On top of that Jon also claims to have a guilty verdict on him.
It's not water-tight, but it's good enough to start with. If Jonnas isn't MI6 then the Mafia got very lucky in using some sort of ability to make Jonnas appear evil, or in tampering with Jon's received PM. In terms of pure probability, it's a 1 in 15 chance of Jonnas randomly targeting the murdered player (assuming he was town, of course), multiplied by a 1 in 15 chance of the Mafia randomly picking the right guy (Jon) to screw with. Which works out as a 1 in 225 chance of just happening randomly.
So either there's decent odds that Jonnas is MI6, some town/neutral members are being silly on the first night (ie. MI6 didn't cause this), or Jon is lying.